188 CONSERVATION OF GROUND WATER 



conservation of soil water are very generally complementary 

 programs. Any benefits to ground-water reservoirs have been 

 largely incidental to these programs. 



In artificial replenishment of ground-water reservoirs, many 

 of the same techniques may be used, but the major objective 

 is different. For the water user, the project is successful only 

 if the replenishment to the ground-water reservoir is actually 

 increased, and the fact of increased infiltration alone is not 

 enough. 



For artificial replenishment of a specific ground-water 

 reservoir, it is necessary first to know that the water is being 

 applied in an area where it can actually enter the reservoir. 

 Special investigation may be required here, because the re- 

 charge areas of most ground-water reservoirs have not yet 

 been delineated or mapped. A second major need is to be 

 certain that the water applied for recharge actually gets into 

 the reservoir and is not dissipated by evapotranspiration from 

 the soil zone. 



Most of the important ground-water reservoirs of the West 

 are in the arid intermontane valleys and receive only a small 

 proportion of their replenishment from precipitation on the 

 valley floor. The bulk of the recharge comes from streams, and 

 the greatest replenishment occurs when the stream flow is 

 greatest. The quantity of water that continues on downstream 

 is also greatest at that time, and in the streams that flow into 

 the Pacific Ocean, Great Salt Lake, or other interior basins, 

 much of this flow is wasted. 



Salvaging of this surplus water has been accomplished in 

 many places by devices that hold the floodwater back and/or 

 divert it into ground-water reservoirs by spreading it over 

 permeable beds, thus causing artificial recharge. The success 

 of water spreading is dependent upon the rate of infiltration 

 that can be maintained, and many difficulties have developed 

 because of sediment carried by the floodwater, or chemical 

 constituents, or other hindrances to infiltration, even in sand 

 and gravel or other highly permeable material. 



